This invention relates to a transfer switch and, more particularly, to a separately-derived transfer switch having a lockout sequencing arrangement that sequences manual switching of a load between power supplies to prevent open neutral transients during the switching.
In an electrical supply system, there are occasions when an alternate source of electric power is necessary or desirable. For example, the capability of switching from utility power to emergency generator power is important for businesses, hospitals and industries, and is also employed in residential applications.
It is desirable for separate electrical circuits, or separate groups of electrical circuits, to be arranged so that when one group of circuits is switched to a conductive state, another group of circuits is switched to a non-conductive state so as to prevent power supply to the circuits from two different power sources at the same time, e.g. from both a utility power supply and a generator power supply. In an arrangement such as this, a switch is typically provided for each power source to control the supply of electrical power. Accordingly, it is important to ensure that the switches are prevented from both being in the ON position at the same time, to ensure that power is supplied to the switch from only one power source.
To this end, switch interlocks have been developed that are designed to prevent simultaneous connection of circuits to two different power sources, such as described in U.S. Pat. No. 6,096,986, the disclosure of which is incorporated herein and assigned to the assignee of the present application. For some transfer switches, providing linkages that prevent the inadvertent switching of circuits to two power supplies is sufficient. However, for some types of transfer switches, more than an interlock is needed. For instance, if a separately-derived transfer switch is not properly switched, open neutral switching transients may be introduced.
The present invention is directed to a sequencing lockout arrangement for use with a separately-derived transfer switch that sequences manual switching of main and generator side switches to prevent the introduction of open neutral switching transients. A separately-derived transfer switch typically includes a utility mains switch or breaker and a utility mains neutral switch as well as a generator mains switch or breaker and a generator mains neutral switch. In one embodiment of the present invention, two slidable lockout sequencers together with a rocker lockout functions to sequence switching of a load from one power source to another power source. In this embodiment, seven separate operations must be performed to switch the load between power sources. In another embodiment, the utility mains neutral and generator mains neutral switches are linked together such that switching of the utility mains neutral to a conductive position automatically switches the generator mains neutral switch to a non-conductive position, and vice-versa. In this embodiment, five separate operations are required to switch a load between power sources.
The slidable lockout sequencers together with the rocker lockout in the first-mentioned embodiment allow only one of the utility mains breaker, the utility mains neutral switch, the generator mains breaker, and the generator mains neutral switch to be switched at a time. Moreover, the lockout sequencers and the rocker lockout cooperate such that a pre-defined order or sequence of the one-at-a-time switching must be followed to switch a load from one power source to another. The slidable lockout sequencers similarly define the sequence of switching with the interlinked neutral switches of the second-mentioned embodiment. Thus, in both embodiments, the slidable lockout sequencers provide limited and ordered switching of the utility and generator switches.
Thus, it is one object of the present invention to provide a lockout arrangement for use with a separately-derived transfer switch that is operable to prevent open neutral switching transients.
It is another object of the present invention to provide a separately-derived transfer switch having a pair of slidable members that restrict movement of switch handles such that a load is switched from one power source to another in a pre-defined, unalterable sequence.
In accordance with one aspect of the present invention, these and other objects are achieved with a lockout arrangement for use with a separately-derived transfer switch having a mains switch, a generator switch, a mains neutral switch, and a generator neutral switch. The lockout arrangement includes a neutral interlock associated with the mains neutral switch and the generator neutral switch, and configured to prevent both neutral switches from being in a conductive position simultaneously. The neutral interlock includes a first bracket engaged with the mains neutral switch and a second bracket engaged with the generator neutral switch. The brackets are adapted to move in response to movement of a neutral switch. The lockout arrangement further includes a first interlock configured to engage the first bracket to prevent movement of the first bracket, and a second interlock configured engage the second bracket to prevent movement of the second bracket. The first and second interlocks are arranged such that the interlocks cannot be engaged with their respective brackets simultaneously.
In accordance with another aspect, the invention is directed to a separately-derived transfer switch having a first mains switch associated with a first power supply and a second mains switch associated with a second power supply. The transfer switch further includes a first neutral switch and a second mains neutral switch associated with the first and the second power supplies, respectively. A lockout sequencing arrangement has a first lockout that restricts simultaneous switching of the first and the second neutral switch and further includes a second lockout configured to engage the first lockout to restrict movement of the first lockout when the first main switch is a conductive position, and a third lockout configured to engage the first lockout to restrict movement of the first lockout when the second main switch is in a conductive position.
The present invention may also be embodied in a method of disconnecting a load from a utility power supply and connecting the load to a generator. The method includes switching a mains switch from an ON position to an OFF position. The method continues with disengaging a mains side lockout from engagement with a neutral switch assembly lockout to allow movement of a mains neutral switch and a generator neutral switch. The method further includes switching, in tandem, the mains neutral switch from an ON position to an OFF position and the generator neutral switch from an OFF position to an ON position. In addition, the method includes engaging a generator side lockout with the neutral switch assembly lockout to prevent switching of the mains neutral switch and the generator neutral switch, and switching a generator switch from an OFF position to an ON position The above series of steps may be performed in a reverse order to disconnect the load from the generator and to connect the load to the utility power supply.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.